In automobiles and various similar vehicles, a constant velocity universal joint is provided on a power transmission path used to transmit driving force from the engine to the wheels. The constant velocity universal joint is capable of transmitting rotational force at a constant speed even when angular displacement and axial displacement occur between two axes.
As shown in FIG. 8A, basic constituent elements of the constant velocity universal joint include an outer joint member 101, an inner joint member 102, a plurality of torque transmitting balls 103, and a cage 104. A plurality of guide grooves 101b are formed on a spherical inner circumferential surface 101a of the outer joint member 101. A plurality of guide grooves 102b are formed on a spherical outer circumferential surface 102a of the inner joint member 102. The torque transmitting balls 103 are disposed on ball tracks formed by opposing guide grooves 101b of the outer joint member 101 and guide grooves 102b of the inner joint member 102. The cage 104 is interposed between the outer joint member 101 and the inner joint member 102, and holds the torque transmitting balls 103. The joint is a fixed-type constant velocity universal joint that does not make plunging movements. A joint center O is fixed regardless of an operation angle. In this instance, six torque transmitting balls 103 are disposed evenly spaced in a circumferential direction.
A center of curvature of the inner circumferential surface 101a of the outer joint member 101 and a center of curvature of the outer circumferential surface 102a of the inner joint member 102 are both aligned with the joint center O. On the other hand, a center of curvature O101 of the guide groove 101b of the outer joint member 101 and a center of curvature O102 of the guide groove 102b of the inner joint member 102 are offset by equal distances F1 and F2 in opposite directions in an axial direction with the joint center O therebetween (in the example shown in FIG. 8A, the center O101 is closer to a joint opening-end side, and the center O102 is closer to a joint closed-end side). Therefore, a ball track formed by a guide groove 101b and a guide groove 102b that oppose each other is formed into a wedge shape that becomes wider in either axial direction.
When torque is transmitted while the joint is at an operating angle, a thrust force M is generated that attempts to push the ball 103 from the narrow portion of the wedge-shaped ball track to the wide portion, as shown in FIG. 8B. As a result, inner and outer circumferential surfaces of the cage 104 press against the inner circumferential surface 101a of the outer joint member 101 and the outer circumferential surface 102a of the inner joint member 102. Friction generated at this time causes rotational torque loss. Magnitude of the thrust force M corresponds to a size of an angle of nip γ that is formed by two tangential lines in the axial direction of the ball 103 in relation to the guide groove 101b and the guide groove 102b. In other words, the thrust force M increases as the angle of nip γ increases.
A joint including eight torque transmitting balls (refer to, for example, Patent Document 1 and Patent Document 2) is known as a joint that is more compact and has higher torque transmitting efficiency than the joint including six torque transmitting balls 103 shown in FIG. 8A. Constituent elements of a constant velocity universal joint including eight balls shown in FIG. 9A are basically similar to those of the constant velocity universal joint shown in FIG. 8A. Sections that are the same are given the same reference numbers. Redundant explanations are omitted. The joint in FIG. 9A has a smaller ball size, and the center of curvature of the guide grooves is offset by a smaller amount, compared to the joint in FIG. 8A.
More specifically, in FIG. 9A, an angle of nip γ′ is smaller than the angle of nip γ in FIG. 8A as a result of an offset F1′ of the center of curvature O101 and an offset F2′ of the center of curvature O102 being smaller than the offset F1 and the offset F2 in FIG. 8A. In other words, as a result of the angle of nip γ′ in FIG. 9A being reduced (γ′<γ), thrust force M′ shown in FIG. 9B is also reduced. Therefore, friction between the contact surfaces of the cage, the inner joint member, and the outer joint member during torque transmission can be reduced.
FIG. 10 shows a six-ball joint. FIG. 11 shows an eight-ball joint. In FIG. 10, A is a contact point path of the ball 103 in relation to the guide groove 102b on the inner joint member 102. B is a contact point path of the ball 103 in relation to the guide groove 101b of the outer joint member 101. In FIG. 11, A′ and B′ are contact point paths similar to those in FIG. 10. An example of length ratios of the contact point path A, the contact point path A′, the contact point path B, and the contact point B′ are shown in Table 1, below, the contact point path A being 1. Respective ball diameter ratios (R:R′) and offset ratios (F1:F1′ or F2:F2′) of the six-ball joint and the eight-ball joint at this time are also shown in Table 1.
TABLE 1Six-ball jointEight-ball jointContact path length ratio in1.53 (A)1 (A′)relation to inner joint memberContact path length ratio in1.61 (B)1.06 (B′)relation to outer joint memberBall diameter ratio1.25 (R)1 (R′)Offset ratio1.68 (F1)1 (F1′)
It is clear from Table 1 that the difference between the contact point path of the inner joint member and the contact point path of the outer joint member in the eight-ball joint (B′−A′=0.06) is smaller than the difference between the contact point path of the inner joint member and the contact point path of the outer joint member in the six-ball joint (B−A=41.08). As a result of a configuration such as this, in the eight-ball joint, sliding speed between the ball and the guide groove on the outer joint member decreases, and torque transmitting efficiency improves.
Therefore, in the eight-ball joint, friction generated at the contact surfaces between the cage, the inner joint member, and the outer joint member can be reduced, compared to the six-ball joint. In addition, because sliding between the ball and the guide groove is reduced, torque can be transmitted with high efficiency.    Patent Document 1: Japanese Patent Publication No. 3460107    Patent Document 2: Japanese Patent Laid-open Publication No. Heisei 9-317784